Numerous different methods are known for recognizing misfires. Their common purpose is to interrupt the supply of fuel to the affected cylinder. The air induced by this cylinder, however, is then still available for combustion in the catalytic converter. This becomes problematic when the other cylinders are operated with a rich mixture. This condition occurs particularly in the case of lambda control when lean mixture is initially indicated because of the excess air from the affected cylinder. In order to avoid the damaging effect of excess air, it is known from DE-A-23 40 541 to arrange a controllable flap in each induction pipe to the individual cylinders of an engine. If the fuel supply to a cylinder is interrupted, the associated flap is at the same time adjusted in such a way that the cylinder is cut off from the air supply. This protective measure, however, involves a substantial structural complexity.
Another measure for protecting a catalytic converter in the case of interrupted fuel supply to one cylinder consists in substantially reducing the power of the engine. In this case, post-combustion of air from the affected cylinder and fuel from the rich mixture in the other cylinders does not lead to overheating of the catalytic converter. Although this method does not require special structural measures, the substantial reduction in power is disadvantageous.
There is, therefore, the long-standing problem of providing a method for protecting a catalytic converter from overheating in the case of misfires in an engine which requires no special structural measures and which does not lead to a substantial reduction in power.